1. Field of the Invention
The present invention relates to a process for producing a vinyl chloride-based polymer which enables a vinyl chloride-based polymer having good anti-initial discoloration to be produced with good productivity.
2. Description of the Prior Art
Vinyl chloride-based polymers are typically produced by conducting a suspension polymerization of either vinyl chloride monomer or a monomer mixture containing vinyl chloride monomer, in an aqueous medium, using an oil soluble polymerization initiator and in the presence of a suspension stabilizer.
Amongst conventional processes for producing vinyl chloride-based polymers, known techniques that have been adopted for improving the polymer yield without lengthening the polymerization time include processes that use large quantities of an oil soluble polymerization initiator, and processes in which the temperature inside the polymerization vessel is raised when the polymerization reaction has progressed substantially and the pressure inside the polymerization vessel begins to fall (for example, Japanese Laid-open Patent Publication (kokai) No. 9-143208 (JP9-143208A)). However, the polymers obtained from these processes tend to be colored slightly yellow when formed into products such as films or the like. In other words, the anti-initial discoloration is poor.
It is known that the anti-initial discoloration varies significantly depending on the type of polymerization initiator used. For example, it is known that the use of an initiator such as di-2-ethylhexyl peroxydicarbonate results in an improved yield but a large deterioration in the anti-initial discoloration of the product polymer, whereas the use of an initiator such as t-butyl peroxyneodecanoate produces good anti-initial discoloration but produces only a small increase in yield. Hence, with conventional processes for producing vinyl chloride-based polymers, it has proved difficult to obtain a vinyl chloride based polymer having good anti-initial discoloration with a high yield.